The present invention is directed to copolymer compositions useful in the preparation of fuel tank sealants. More particularly, the present invention relates to condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer compositions and the use of polyalkoxysilylorganic compounds as cross-linkers to facilitate the condensation cure of poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymers.
As shown by Smith, U.S. Pat. No. 3,109,826, bis(alkoxysilyl) hydrocarbons, such as 1,2-bis-(triethoxysilyl)ethane, can be used as cross-linkers in combination with a metal salt to effect the neutral condensation cure of hydroxy end-blocked polydiorganosiloxanes. However, the resulting cured silicone compositions have been found to be problematic as aircraft fuel tank sealants, as they do not have the required solvent or fuel resistance. It is known that fluorosilicones, for example, made by polymerizing tris[(trifluoropropyl)methyl]cyclosiloxane, can provide excellent fuel resistance. However, fluorosilicones often do not meet the wide temperature stability requirements needed in aircraft sealants, such as temperatures in a range between xe2x88x9254xc2x0 C. and 177xc2x0 C. over an extended period of time. In addition, fluorosilicones are subject to depolymerization which can result in the formation of low molecular weight cyclics.
In an effort to enhance the thermal stability of fluorosilicones, non-siloxane groups, such as p-silphenylene, have been inserted into the polyfluorosiloxane backbone, as shown by Grassie and Beattie, xe2x80x9cThe Thermal Degradation of Polysiloxanes: Part 7xe2x80x9d, Polymer Degradation and Stabilization 8:177-193 (1984). It is also reported by Dvornic and Lenz, Macromolecules, 25, 3769 (1992), that copolymers having a glass transition temperature (Tg) of xe2x88x9251xc2x0 C. can be made by reacting methyl(3,3,3-trifluoropropyl)silanediol and 1,4-bis(dimethylhydroxysilyl)benzene.
While fluorosilicones having improved thermal stability have been made by inserting non-siloxane groups, such as p-silphenylene into the polyfluorosiloxane backbone, such copolymers have been found to have a glass transition temperature which does not satisfy the minimum xe2x88x9254xc2x0 C. Tg flexibility requirements of aircraft fuel tank sealants.
Experience also has shown that in addition to being sensitive to depolymerization, condensation curable fluorosilicone compositions often suffer from an incomplete cure using a conventional curing catalyst, such as a tin salt, and a standard neutral condensation curable cross-linker, for example a polyalkoxysilane. One possible explanation, as discussed by Fujiki, U.S. Pat. No. 5,236,997, is that the steric hindrance or electronic effects of bulky terminal trifluoropropyl groups inhibit crosslinking of the network.
Accordingly, depolymerization resistant silicone base copolymers which could be compounded to a fuel resistant condensation curable silicone composition convertible to the elastomeric state upon cure exhibiting stability over an operable temperature in a range between about xe2x88x9254xc2x0 C. or below and at least about 177xc2x0 C. over an extended period of time are constantly being sought which also exhibit low temperature flexibility, in addition to high temperature stability.
The present invention provides a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer exhibiting a glass transition temperature not exceeding about xe2x88x9254xc2x0 C.
A further embodiment of the present invention provides a method for making a poly(fluoroorgano)siloxane-poly(silarylene)siloxane copolymer comprising effecting reaction between a bis(diorganohydroxysilyl)arylene and a poly(fluoroalkylorgano)cyclopolysiloxane
In yet a further embodiment of the present invention, there is provided a neutral condensation curable poly(fluoroorgano)siloxy-poly(silarylene)siloxane block copolymer sealant composition comprising
(a) a poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) a cross-linker, and
(c) a condensation catalyst.
In yet another embodiment of the present invention, there is provided a method for making a neutral condensation curable poly(fluoroorgano)siloxy-poly(silaylene)siloxane block copolymer sealant composition which comprises
(a) effecting reaction between bis(diorganohydroxysilyl)arylene and poly(fluoroalkylorgano)cyclopolysiloxane to form a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) shearing the copolymer, and
(c) blending a cross-linker and a condensation catalyst with the copolymer to form a sealant.
The poly(fluoro)organosiloxane-poly(silarylene)siloxane block copolymers, or xe2x80x9cblock copolymersxe2x80x9d within the scope of the present invention, can be used to make aircraft fuel tank sealants having property profiles which include a glass transition temperature (Tg) of about xe2x88x9254xc2x0 C. or below and can be made by the ring opening polymerization of a poly(fluoroalkylorgano)cyclopolysiloxane in the presence of a bis(diorganohydroxysilyl)arylene.
As used hereinafter, the term xe2x80x9cbis(diorganohydroxysilyl)arylenexe2x80x9d, or xe2x80x9cbis(hydroxysilyl)arylenexe2x80x9d is shown by the formula,
(HOxe2x80x94(R)2Si)2xe2x80x94Q1,xe2x80x83xe2x80x83(I)
and the term xe2x80x9csilarylenesiloxyxe2x80x9d can be represented by the formula,
xe2x80x94(R)2SiQ1Si(R)2Oxe2x80x94,xe2x80x83xe2x80x83(II)
where Q1 is a C(6-12) divalent aromatic organic radical, and R is a C(1-4) alkyl radical. Preferably, the bis(diorganohydroxysilyl)arylene is 1,4-bis(dimethylhydroxysilyl)benzene.
xe2x80x9cPoly(fluoroalkylorgano)cyclopolysiloxanexe2x80x9d, sometimes expressed as xe2x80x9cpoly(fluoroalkyl)cyclic siloxanexe2x80x9d is shown by the following formula:
[(R1)(R2)SiO]a,xe2x80x83xe2x80x83(III)
where R1 is a C(3-8) polyfluoroalkyl radical, R2 is a C(1-12) organic radical, and xe2x80x9caxe2x80x9d is an integer in a range between about 3 and about 8 inclusive, and preferably 3 or 4. Poly(fluoroalkylorgano)cyclopolysiloxane can be used in a proportion in a range between about 0.5 and about 4 moles, and preferably in a range between about 1 and about 2 moles, per mole of bis(diorganohydroxysilyl)arylene. Reaction to form the copolymer is typically effected with agitation under neat conditions or in the presence of an inert organic solvent, for example, toluene, at temperatures in a range between about 60xc2x0 C. and about 150xc2x0 C. Reaction is typically effected for a time period in a range between about 30 minutes and about 2 hours, and preferably, in a range between about 45 minutes and about 1.5 hours. It is preferred to operate within an inert atmosphere, for example, under a nitrogen blanket. Typically, there is also present an initiator, for example, an alkali hydroxide (e.g. sodium hydroxide) or an alkali fluorosilanolate (e.g. sodium fluorosilanolate) in a range between about 5 parts per million (ppm) and about 50 ppm, based on the weight of reaction mixture, and a quencher, for example, phosphoric acid or silyl phosphate in a range between about 10 ppm and about 60 ppm, based on the weight of reaction mixture.
C(1-4) alkyl radicals which are included within R are, for example, methyl, ethyl, propyl and butyl; C(3-8) polyfluoroalkyl radicals which are included within R1, include but are not limited to, for example, trifluoropropyl units, tridecafluoro- 1,1,2,2-tetrahydrooctyl units, nonafluoro-1,1,2,2-tetrahydrohexyl units, and pentafluoro-1,1,2,2-tetrahydrobutyl units. Radicals included within R2 include, but are not limited to, for example, methyl, ethyl, propyl, butyl, and phenyl.
Among the poly(fluoroalkylorgano)cyclopolysiloxanes there is preferably included tris[(trifluoropropyl)methyl]cyclosiloxane.
The sealant composition of the present invention includes a bis(polyalkoxysilyl)organo cross-linker having the formula,
[(RO)2(X)Si]2Q,xe2x80x83xe2x80x83(IV)
where Q is a C(2-12) divalent organic radical, R is as previously defined, and X is a member selected from the group consisting of R and RO. Some of the C(2-12) divalent hydrocarbon radicals included within Q are dialkylene radicals such as dimethylene, trimethylene, tetramethylene, pentamethylene, and hexamethylene. Also included are divalent arylene radicals, for example, phenylene, tolylene, xylylene and naphthylene. Divalent aromatic organic radicals included within Q are for example phenylene, tolylene, xylylene and naphthylene.
The cross-linker can be used in combination with an effective amount of a condensation catalyst, such as a tin compound, to effect the neutral condensation cure of block copolymers comprising blocks of poly[(fluoroorgano)organo]siloxy units chemically combined with poly(silylarylene)siloxane groups. The block copolymers are referred to as xe2x80x9cpoly(fluoro)organosiloxane-poly(silarylene)siloxane block copolymersxe2x80x9d.
The sealant composition of the present invention is preferably made by initially compounding the poly(fluoroorgano)siloxane-poly(silarylene)siloxane copolymer, or xe2x80x9cblock copolymerxe2x80x9d into a curable sealant base, or masterbatch under shearing conditions. Shearing can be effected at a temperature in a range between about 25xc2x0 C. and about 200xc2x0 C., preferably, in a range between about 100xc2x0 C. and about 150xc2x0 C., at atmospheric pressure, and preferably under inert conditions, such as under an inert gas, for example nitrogen, in a mixer, for example, a double-planetary mixer. Shearing can be carried out over a period in a range between about 15 minutes and about 4 hours, preferably in a range between about 1 hour and about 2 hours.
The block copolymer can be blended with a filler which can include a reinforcing filler, such as fumed silica, or a combination thereof with an extending filler, such as diatomaceous earth, precipitated silica, ground quartz, or calcium carbonate. It is preferred to use fumed silica which has been pretreated with an effective amount of a cyclic siloxane, such as octamethylcyclotetrasiloxane, or a mixture thereof with an organosilazane, such as hexamethyldisilazane. A proportion of filler in a range between about 0 parts and about 30 parts by weight, per 100 parts of block copolymer can be used. Preferably, the filler is present in a range between about 5 parts and about 15 parts by weight per 100 parts of block copolymer. A heat stabilizer, such as iron oxide, in a range between about 0.1 and about 10 parts by weight, per 100 parts of block copolymer has been found to be effective.
After blending of the masterbatch ingredients, the mixture can be degassed under reduced pressure, and the resulting paste can be stored. The neutral condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane copolymer sealant compositions can be made by blending the block copolymer masterbatch with an effective amount of a condensation catalyst and a suitable bis(polyalkoxysilyl)organo cross-linker, referred to hereinafter sometimes as xe2x80x9ccross-linkerxe2x80x9d. xe2x80x9cNeutralxe2x80x9d as used herein refers to a sealant composition which is substantially acid-free and substantially base-free.
Suitable condensation catalysts are present in a range between about 0.1 and about 2 parts, per 100 parts of block copolymer and include, for example, organo-metal compounds such as dibutyltin diacetate, dimethyltin neodecanoate, dibutyltin dilaurate, stannous octoate, dimethyltin hydroxyoleate, or combinations thereof.
Some of the preferred bis(polyalkoxysilyl)organo cross-linkers included within formula (1) are, for example, 1,2-bis(triethoxysilyl)ethane, 1,6 bis(trimethoxysilyl)hexane, 1,4-bis[trimethoxysilyl(ethyl)]benzene, 1,2-bis(methyldiethoxysilyl)ethane, and 1,6-bis(methyldiethoxysilyl)hexane. The cross-linkers are present in a range between about 1 and about 20 parts, per 100 parts of block copolymer.